This disclosure generally relates to apparatus and methods for non-destructive inspection (NDI) of structural elements and, more particularly, relates to NDI techniques for detecting fatigue cracks around fasteners.
Non-destructive detection and evaluation of stress-induced fatigue cracks in metals may be practiced in many different environments, including surface transportation, aerospace transportation and power plants. For example, eddy current testing may be used to identify cracks that may not be visible. In some cases, paint may be removed to perform an inspection. Some paints or coatings have a conductive material that may make it more difficult to identify cracks when eddy current testing is used. Eddy current testing uses electromagnetic induction to identify cracks in conductive materials, such as metal skin panels. In particular, eddy current testing near features, such as fasteners, is affected by the electrical conductivity differences between the structure and the fastener. This difference may limit the sensitivity of this type of testing to detect inconsistencies. These types of inspections may require more time and expense than desired.
Another known technique for detecting cracks in metals uses a near-field millimeter wave (i.e., a wavelength range of 1-10 mm) waveguide probe. Millimeter wave signals do not penetrate through metals but are sensitive to the presence of metal surface discontinuities such as cracks. Advantageously, millimeter wave signals are able to propagate through dielectric materials, such as paint. Thus a waveguide probe can interrogate paint-covered metal surfaces. If a crack is present in the interrogated volume, the crack will produce a perturbation in the surface current density induced in the waveguide probe.
A known method of detecting cracks in metal around fasteners uses a hand-held waveguide probe. It would be desirable to provide an automated apparatus capable of performing millimeter wave crack detection, enabling crack detection that is faster, less labor intensive, more repeatable, and ergonomically safer than using a hand-held waveguide probe.